The present invention relates to voltage stabilizer devices and in particular to monolithically integrated voltage stabilizers which can be used in automobile applications or for portable apparatus.
Voltage stabilizers supply a voltage having a well-specified and constant value or values from a voltage having an indeterminate value which is supplied to them.
Voltage stabilizers can be advantageously used as supply devices for other devices; they supply the current required as a function of the load connected to them so that the voltage supplied to this load always remains constant.
At present, for reasons of compactness, ease of use and economic viability, the voltage stabilizers produced for all fields of application tend to be of the electronic integrated circuit type.
In general, the magnitudes of the voltage and current at the output terminals of these electronic voltage stabilizers are determined by an internal regulation circuit comprising a feedback circuit means connected to the output terminals and sensitive to the instantaneous value of these magnitudes.
The lower limit of the field of correct operation of an electronic voltage stabilizer is ascertained from a parameter which is generally known in the technical literature by the term "drop-out", which is the difference between the minimum value of the input voltage required for correct operation of the stabilizer and the value of the constant output voltage which the stabilizer has to supply and thus indicates the voltage drop across the device. For instance, the voltage stabilizers used in automobile applications have to meet very severe requirements as a result of operating conditions which may involve both substantial temperature and humidity variations and substantial, occasionally abrupt, variations in the supply voltage supplied by the motor vehicle battery.
These stabilizers must therefore be very reliable, accurate and stable, while at the same time being economically viable, and must in particular have a low drop-out since the supply voltage supplied by the battery of a motor vehicle may normally drop, during cold starting, from the typical 14.4 V at full charge to some 6 V.
Account must also be taken of the positive and negative voltage peaks having a maximum amplitude of up to 150 V which may be present on the supply line of a motor vehicle as a result of the switching transients of inductive loads (ignition coils, relays, etc.) or electrical connection cable detachments or breakages.
The monolithically integrated voltage stabilizer circuits most commonly used for automobile applications are those with so-called "series"-type regulation, in which the output voltage is regulated to a constant value by a bipolar power transistor connected in series to an output terminal and suitably base-controlled to cause it to conduct as a function of the load.
A suitably dimensioned power transistor may even withstand, with no drawbacks, positive voltage peaks having a high amplitude and thus continue to ensure the regulation of the output voltage.
The negative peaks of the input voltage could, however, cause the transistor to be cut off, thereby causing interruptions, albeit brief, in the supply to the consumer circuits connected to the voltage stabilizer, with serious drawbacks when these comprise integrated memories and logic circuits which require a constant supply.
For this reason, voltage stabilizers comprising "series"-type regulation circuits also comprise a capacitor and an input diode, which are not integrated, so that a sufficient supply to the power transistor can be maintained during very short negative transients in the input voltage.
FIG. 1 of the drawings shows the known diagram of a voltage stabilizer with "series"-type regulation obtained by PNP power transistors.
The circuit diagram of FIG. 1 comprises a bipolar PNP transistor T having its emitter terminal connected to the cathode of a diode D, whose anode forms an input terminal IN, and to a first terminal of a capacitor C whose second terminal is connected to ground. The collector terminal of the transistor T forms an output terminal OUT.
The base terminal of the transistor T is connected to the output terminal of a differential amplifier A whose non-inverting input is connected via a first resistor R.sub.1 to the terminal OUT and is connected via a second resistor R.sub.2 to ground.
The inverting input of the amplifier is, in contrast, connected to a voltage reference V.sub.R.
The part of the diagram of FIG. 1 which represents the voltage regulator circuit which can be monolithically integrated is enclosed in a rectangular block of dashed lines.
The capacitor C is charged via the diode D to the typical values of the battery voltage less the voltage drop across the diode itself, during normal charging conditions.
However, during negative voltage transients the diode D prevents the capacitor C from discharging via the input terminal with the result that this capacitor can discharge only via the transistor of the regulation circuit, allowing it to conduct during the transient itself.
In the case of a stabilizer comprising a PNP power transistor, there is a drop out: EQU V.sub.DROP =V.sub.D +V.sub.CE sat
in which V.sub.D is the voltage drop across the diode D when conducting and V.sub.CE sat is the collector-emitter voltage of the transistor T when it is at saturation.
Using an NPN power transistor, it is possible to achieve, with the same drop-out, an integration area occupation on the part of the regulation circuit which is lower than that which can be obtained with a PNP power transistor.
FIG. 2 shows the diagram of a voltage stabilizer comprising a bipolar NPN power transistor T'.sub.1 whose collector terminal is connected to the cathode of a diode D' and to a first terminal of a capacitor C', the second terminal of which is connected to ground.
The circuit diagram also comprises first and second bipolar PNP transistors T'.sub.2 and T'.sub.3, both having their collector terminals connected to the base terminal of the transistor T'.sub.1. The emitter terminal of the transistor T'.sub.2 is connected to the cathode of the diode D' and the emitter terminal of the transistor T'.sub.3 is connected to the anode of the diode D' in a circuit node which forms an input terminal IN' of the stabilizer.
The emitter terminal of the transistor T'.sub.1 forms an output terminal OUT'.
The base terminal of the transistor T'.sub.1 is connected to the output terminal of a differential amplifier A' whose inverting input is connected to the output terminal OUT' via a first resistor R'.sub.1 and is connected to a common terminal GND' via a second resistor R'.sub.2. This common terminal GND' is connected to ground.
The non-inverting input of the differential amplifier is connected to a voltage reference V'.sub.R.
The base terminal of the transistor T'.sub.2 is connected to the common terminal GND' via a first constant current generator G'.sub.2 and is connected to the cathode of a diode D'.sub.2 whose anode is connected to the emitter terminal of the transistor T'.sub.2.
The base terminal of the transistor T'.sub.3 is connected to the common terminal GND' via a second constant current generator G'.sub.3 and is connected to the cathode of a diode D'.sub.3 whose anode is connected to the emitter terminal of the transistor T'.sub.3.
The regulation circuit which can be monolithically integrated is also enclosed in a rectangular block of dashed lines in FIG. 2.
The drop-out of the voltage stabilizer described here has a value: EQU V.sub.DROP =V.sub.BE +V.sub.CE sat'
in which V.sub.BE is the base-emitter voltage of the transistor T'.sub.1 in conduction, with a value approximately equal to the voltage drop V.sub.D at a diode and V.sub.CE sat' is the collector-emitter voltage of the transistor T'.sub.3 when it is at saturation, this drop-out consequently being equal to that of the stabilizer shown in FIG. 1.